This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Background: Combination of 50mM MIV-150 and 14mM zinc acetate dihydrate provides complete protection from infection for at least 24h after application. We hypothesized that the MIV-150/zinc acetate combination gel will protect against SHIV RT infection in macaque vaginal mucosal explants. Methods: Non-polarized vaginal explants (biopsies or necropsy tissues derived from naive and recycled SHIV RT exposed/infected animals) were stimulated with PHA/IL-2 before infection in vitro with SHIV RT. Anti-viral activity of MIV-150 and zinc acetate formulated in carrageenan was tested and compared with activity of gels containing only zinc acetate or MIV-150. Tissue infection was monitored using p27 ELISA and SIVgag PCR. Results: Overnight transported PHA/IL-2 stimulated macaque vaginal mucosa is susceptible to infection with SHIV RT in vitro. PHA/IL-2 stimulated tissues not exposed to virus in vitro were included in all experiments as negative controls. Infection of explants in the presence of the MIV-150/zinc acetate combination gel, as well as MIV-150 or zinc acetate only containing gels, resulted in complete protection against infection. Infection in the presence of carrageenan-based vehicle resulted in delayed infection kinetics or complete protection. This protective effect of vehicle was overcome when tissues were exposed to a higher dose of SHIV RT. Tissues challenged 24h after exposure to the MIV-150/zinc acetate containing gel did not become infected (memory effect), while tissues pretreated with the carrageenan vehicle were infected like no gel treated controls. Experiments comparing the memory effect of the gels are ongoing. Conclusion: A novel MIV-150/zinc acetate combination gel (but not the carrageenan vehicle) is effective against SHIV RT in macaque vaginal explants when applied 24h prior to virus exposure. This mirrors what was observed in vivo following vaginal application supporting the use of this explant system to prescreen candidate microbicides prior to in vivo animal testing.